You have to work this out step-by-step.
Population started at 1000.
It went up 10%: 1000 + 10%(1000) = 1000+100 = 1100
Now it's 1100
It then went down 20%: 1100 - 20%(1100) = 1100 - 220 = 880
Now it's 880.
It then went back up 10%... I'll let you try the last calculation.
Can you take it from there?